Specificity of neuronal function at the cellular level is dictated by the exact functional properties of channels and receptors and the precise spatial distribution of these molecules. This specificity is vital to systems-level function. For example, severe loss of motor ability occurs in response to disorganization of Na channel distributions in demyelinating diseases such as multiple sclerosis and as a result of mutated Na channel genes that perturb specific functional properties in hyperkalemic periodic paralysis and related myotonias. Other neuromuscular and central disorders are likely to involve related mechanisms, but the molecular and cell biological dynamics that control function and distribution of channels and receptors are not well understood. This is true even for the Na and K channels responsible for a fundamental function like action potential conduction, especially in light of the fact that both of these "channels" represent multi-gene families. At least 7 Na channel mRNA species are expressed in skeletal muscle. Four distinct sub- families of K channel genes exist; at least 10 human members of just the Shaker-related subfamily 1 have been reported. Expression of cloned channels in heterologous systems reveal a broad array of functional differences, even between closely related subtypes. How much of this complexity exists in vivo at the level of a single neuron, and to what degree can we understand the complexity of a single cell given available technology? These fundamental questions demand answering. This proposal describes a systematic approach to this end by focusing on the Na and Shaker-like K channels responsible for impulse conduction in a single neuron, the squid giant axon and its cell bodies. Unique features of this system, coupled with the enormous data base on functional properties of squid Na and K channels, will lead to a deep understanding of the processes acting to control neuronal function in a biologically relevant context. Specific aims will answer: What channel subtypes are expressed and how do their native functional properties compare to those for their cloned counterparts? What is the spatial distribution of each channel isotype? How do post-translational modifications and interactions with other proteins act to control these spatial distributions and functional attributes? Finally, how are these control processes involved in long-term modification of expression patterns for specific channel isotypes that occur during development or in acquisition of new firing patterns? Answers to these questions will significantly advance our understanding of maintenance and modulation of neuronal function in living systems.